A polarization conversion device is an optical device which converts a polarization state of an input light into a different polarization state to output the converted light, for example, for when an input light of TM mode is converted into a light of TE mode to be output. In the case where such a polarization conversion device is made to be a reflective type in which an input section and an output section are common, it may be considered to configure this polarization conversion device using a ¼ wave plate and a reflective mirror, as illustrated in FIG. 9 of Japanese Laid-open Patent Publication No. 2007-163780. In this reflective type polarization conversion device, the light of TM mode (or TE mode) input from the input section passes through the ¼ wave plate, and thereafter, is reflected by the reflective mirror to again pass through the ¼ wave plate, so that the light of TM mode (or TE mode) is converted into the light of TE mode (or TM mode), to be output from the input section.
However, in the polarization conversion device of the above-mentioned configuration, there is a problem in that, when there is generated a component which has not been polarization-converted due to deviation in a setting angle of the ¼ wave plate and in thickness thereof, wavelength dependence thereof, a temperature state thereof or the like, the light containing the component which has not been polarization-converted and a component which has been polarization-converted is output, and therefore, a polarization extinction ratio of the output light is degraded.